Melt spinning process

ABSTRACT

A METHOD FOR DECREASING THE BIREFRINGENCE OF A MELT SPUN FILAMENT BY APPLYING A FORWARDING FORCE TO A FILAMENTARY STREAM IN A REGION WHERE THE FILAMENT IS IN AN INTERMEDIATE PLASTIC STATE, THE FORWARDING FORCE BEING SUFFICIENTLY SMALL SUCH THAT THE FILAMENT IS UNDER POSITIVE TENSION THROUGHOUT THE PROCESS SEQUENCE.

Dec. 19, BREMNER ETAL MELT SPINNING PROCESS 2 Sheets-Sheet 1 Filed May21, 1970 Dec. 19, 1972 D. BREMNER ETAL $706,826 H m MELT SPINNING PROCESS Filed May 21, 1970 2 SheetsSheet 2 32 W 1 Q 33 N P /34 INVENTORS JAMESDUNCAN REMNER IAI QJ ALEXAND MCLELLAN FW "(W7 United States Patent3,706,826 MELT SPINNING PROCESS James Duncan Bremner and Iain AlexanderMcLellan, Harrogate, England, assignors to Imperial Chemical IndustriesLimited, London, England Filed May 21, 1970, Ser. No. 39,525 Claimspriority, application Great Britain, May 23, 1969, 26,429/ 69 Int. Cl.B28b 3/20 US. Cl. 264-176 F 6 Claims ABSTRACT OF THE DISCLOSURE A methodfor decreasing the birefringence of a melt spun filament by applying aforwarding force to a filamentary stream in a region where the filamentis in an intermediate plastic state, the forwarding force beingsufficiently small such that the filament is under positive tensionthroughout the process sequence.

This invention relates to melt spinning processes for aromaticpolyesters in which 'a fluid stream of polymer is extruded at acontrolled speed from a spinnert, and cooled to form a solidifiedfilament which is hauled oif at a higher controlled speed; suchfilaments being subsequently drawn.

in such spinning processes the filamentary streams have to stretchsomewhere in the threadline between the initial extrusion speed and thefinal higher haul olf speed. Such stretching tends to orient the polymermolecules, and after solidification some degree of orientation usuallyremains which may be measured in the resultant filaments as an opticalbirefringence. This birefringence increases with increasing haul oifspeed, and the ratio to which a filament may subsequently be drawndecreases with increasing birefringence, with the unfortunateconsequence that if it is desired to make a drawn filament of a certaindenier, then any increase in productivity deriving from a higherspinning speed is at least partially offset by the need to spin a lowerdenier and draw to a lower ratio. There are'also circumstances, forexample in the production of high tenacity filaments, where a lowbirefringence in the spun filaments is beneficial quite apart fromproductivity considerations.

It is known that increasing extrusion temperature and decreasing coolingrate can decrease the birefringence, but these effects are not sutncientto counter the effect of haul off speed and permit the production offilaments with low birefringence at high haul olf speeds.

After extrusion, filamentary streams cool rapidly but remain molten fora substantial initial distance from the spinneret, within which regionany chance contacts cause them at least temporarily to coalesce. -Inthis region the bulk of the stretching occurs. This molten region isfollowed by a short plastic region in which the viscosity rises betweenthe previous molten region and the subsequent solidified region. In thisplastic region occasional contact between filaments does not cause theproblems due to coalescence which characterise the molten region butsome stretching continues and the bulk of the birefringence isintroduced. The plastic region is followed by a solidified region inwhich no significant further stretching or birefringence changes occur.

According to the present invention we provide a method of decreasing thebirefringence of such melt spun and solidified aromatic polyesterfilaments comprising:

(1) applying a forwarding force to such filamentary streams in a regionin which they are in such mtermediate plastic state Patented Dec. 19,1972 We find that, within the intermediate plastic region, the filamentsare so nearly solidified that they are not very prone to coalescence,and it is possible to apply the force according to this invention by asmall arc of contact round one 101' more driven rollers preferably withroughened surfaces, or by a fluid force applicator, either gaseous orliquid even though each method tends to increase filament contactsbefore convergence in the solidified region. An air ejector device isparticularly convenient as a force applicator, providing an increasedtension upstream in the threadline and a decreased tension downstream,the ejector action being readily controllable to keep the downstreamtension positive in the interests of process control and uniformity.

The air used in such an ejector device may be, but need not be heated.The rate of heat transfer between filaments and a roller is however muchhigher and a driven roller forwarding device should normally thereforebe heated to maintain the filament temperature in the requiredintermediate plastic range.

'Melt spinning processes using ambient cooling, delayed cooling andaccelerated cooling are all known. The present invention may be appliedto each of these types of spinning process and indeed delayed andaccelerated cooling may with benefit be used in sequence in the samethreadline. In one form of the process of this invention, threadlinecooling is delayed between the spinneret and the region in which theforce is applied, and threadline cooling is accelerated thereafter.Using an air ejector as a force applicator it is particularly convenientto use the ejector also to provide accelerated cooling. A surprisingaspect of this invention is that, although a more broadly appliedincreased cooling rate normally increases the birefringence a locallyapplied high rate of cooling coupled with forwarding action can reducebirefringence.

For the better understanding of this invention, some specific exampleswill now be described by way of illustration with reference to theaccompanying drawings in which FIG. 1 is a schematic diagram of a meltspinning process according to the invention and FIGS. 2 and 3 arecross-sections of suitable forms of air ejector.

In FIG. 1, a spinning pack 11 with a spinneret 12 at its base extrudesfilamentary streams 13 which at first are subject to delayed cooling inthe region 14 which is partially enclosed in the thermostated box 15used to heat the pack 11. An air ejector 16 surrounds the threadline ata point between the spinneret 12 and a spin finish applicator 17. Thefilaments are wound up on a bobbin 18. FIG. 2 illustrates one suitableform of air ejector. A short wide bore venturi tube 21 is held by ascrew thread in a. casing member 22 with an air inlet duct 23 connectedto a compressed air supply line 24 by a valve 25. Air is distributedround the venturi tube in the chamber 26 and escapes through annularslot 27.

In operation a high speed inward air flow through the annular slot 27tends to bend downwards parallel with the wall 28 of the venturi tube21, thus entraining with it air in the upper throat 29 of the venturitube 21. This entrained air in turn applies a downward force tofilaments passing through the venturi tube.

FIG. 3 illustrates another suitable form of air ejector. Two members 30and 31 are engaged in a threaded bore in a member 32, forming an annularchamber 33 connected by an annular passageway 34 to a yarn passageway35. Compressed air is fed from means not shown through a bore 36 inmember 32. In operation compressed air passes through the passageway 34to apply a downward force on a yarn in passageway 35. The followingexamples illustrate but do not limit the invention.

EXAMPLE I Using the apparatus of FIGS. 1 and 2, polyethyleneterephthalate with an intrinsic viscosity of 0.67 in orthochlorophenolat 25 C. was extruded at 288 C. through spinneret holes each 9 thou.diameter to form 40 filaments which were wound up at 4140 feet perminute as 8 denier filaments. With no ejector present the filamentbirefringence was 0.0073. When the ejector was positioned 15" from thespinneret, using air pressures of 2, 6 and p.s.i.g. the resultantfilament birefringence was reduced to 0.0059, 0.0049 and 0.0032respectively. Operat ing the ejector at 10 p.s.i.g. it was thenpositioned at 15, 20 and 27" below the spinneret, the resultantbirefringences in each case being 0.0032, 0.0040 and 0.0067

respectively.

EXAMPLE II Polyethylene terephthalate with an intrinsic viscosity of0.67 in orthochlorophenol at 25 C. was extruded at 290 C. throughspinneret holes each 9 thou. diameter to form 20 filaments which werewound up at 4800 f.p.m. as 8 denier filaments. The apparatus of FIG. 3was placed 19 inches below the spinneret.

It was found that the variability of filament denier depended on theyarn passagway length and diameter and best results were achieved with ajet diameter in member 30 or 0.5 inch, a passageway 35 of total length1.25 inches and an air passageway 34 of width 0.01 inch. The airpressure in chamber 33 was adjusted to provide an air pressure reductionof 5.4 cm. water at a point inch above the entrance to passageway 35.

Under these conditions a coefiicient of denier variability of 0.8% wasachieved at a birefringence of 0.0073 compared with 0.5% and 0.010without the ejector present.

EXAMPLE III In place of an air ejector, two driven unheated capstanswere positioned 17.5 inches below the spinneret. Polyethyleneterephthalate with an intrinsic viscosity of 0.67 was extruded at 295 C.to form five filaments wound up at 3700 f.p.m. at a denier of 12. Whenthe threadline passed the capstans without touching them the resultantbirefringence was 0.0085. When the threadline took one complete turnround the capstans rotating at 4075 f.p.m. the birefringence fell to0.0080. Since the capstan speed was slightly higher than the wind upspeed it will be clear that a downward force was applied to thethreadline but the tension between the air ejector and the wind up meanswas positive.

This relatively small reduction in birefringence may be increased byheating the driven capstans.

A melt spinning threadline modified according to this invention isparticularly advantageous in processes involving separate spinning anddrawing, such as are common in the production of staple fibre. In theseprocesses, the productivity advantage of a high spinning speed can begreatly enhanced by the suppression of spun filament birefringenceaccording to this invention. The

invention is however, also useful in integrated spin draw processesbecause it permits control of birefringence other than by first take uproll speed, so that birefringence and draw ratio may 'be adjusted atwill.

What we claim is:

1. In a process for spinning an aromatic polyester comprising (a)extruding a filamentary stream at a controlled speed in the moltenstate,

(b) cooling such stream from its initially molten state through anintermediate plastic state to a solid state, and

(c) hauling off such solidified filament at a higher controlled speed amethod of reducing the optical birefringence of such solidified filamentcomprising (i) applying a forwarding force to such filamentary stream ina region in which it is in such intermediate plastic state (ii) theforwarding force being sufficiently small such that the filament tensionbefore hauling off is always positive.

2. A method according to claim 1 in which the forwarding force isapplied by at least one driven capstan.

3. A method according to claim 2 in which such capstan is heated.

4. A method according to claim 1 in which the forwarding force isapplied by a fluid drag.

5. A method according to claim 4 in which the for-' warding force isapplied by an air ejector.

6. The method of claim 1 wherein said polyester is polyethyleneterephthalate.

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